I think the CD-19 and CD-20 are interchangable, trav. <br /><br />Don't know about the CD-21. 1964 was the year OMC switched oil mixes from 24:1 to 50:1, so there may be some incompatibility.<br /><br />Good luck, and welcome to iboats.
They will all interchange. It's not a bad idea to run any of the 5.5s on a richer mix of gas/oil, as they still have some bronze bearings internally that like the extra oil bath...<br />- Scott
